Conventional food and environmental monitoring relies on laboratory-based biochemical methods, such as polymerase chain reaction (PCR) and enzyme-based immunosorbent assays (ELISA). While these methods are well-established and possess high sensitivity and specificity, they are usually time-consuming (a few days, including sample transportation and analysis), involve laborious processes, and need skilled personnel to perform the analysis. Moreover, such processes usually require sophisticated facilities and strict cleaning standards, prohibiting operation in adverse field conditions. Owing to these challenges, chemical and biological sensors that integrate elements for chemical and biological recognition, respectively, and signal transduction have emerged as robust and state-of-the-art detection techniques for food- and water-borne pathogens.
In sensors, chemical or microbial pathogens are usually recognized by target specific interactions, such as antibody-antigen binding and DNA hybridization, which are then identified by electrochemically, optically, piezoelectrically, or thermally sensitive devices. Enabled by the recent advances in nanomaterials and nanotechnologies, miniaturization of sensing devices has achieved lab-on-a-chip designs that enable portable, cost-effective biosensors that are capable of near real-time (in a few minutes) detection under field conditions. Despite the progress that has been made, a universal challenge for existing sensing technologies is their relatively low sensitivity (e.g., >10 colony-forming units (cfu)/mL) compared to laboratory-based chemical and biochemical methods. Such relatively low sensitivity is primarily due to the reduced device sizes that have limited the number of targets for recognition. This challenge has placed substantial problems on using sensors for detection of pathogens at ultralow concentrations (i.e., <10 cfu/mL).
To approach or achieve high sensitivity, current sensors usually require an additional enrichment stage prior to detection, which can extend the processing time by hours. In contrast, molecularly functionalized magnetic nanoparticles are able to capture pathogenic chemicals or organisms by magnet in batch processes, with demonstrated detection at concentrations as low as approximately 4 cfu/mL. Moreover, magnetic beads have been widely used as labels for genetic detection by using magnetometers, such as superconducting quantum interference device (SQUID), magnetoresitive sensors, and diagnostic magnetic resonance assays. The weak magnetic dipole interactions, however, have limited the sensitivity (e.g., up to approximately 100 cfu for a stream of 10 μL sample) of magnetic sensors. Meanwhile, the commercially viable magnetic probes usually comprise particles of micrometer-scale sizes. Such large probes have limited the number of particles of uptake per pathogen, leading to poor specificity and detection limits.